1. Field:
The present invention relates generally to light emitting diode (LED) based light sources, and more particularly to LED based light sources for fiber-optic applications.
2. Related Art
Fiber-optic light sources are generally well known and are used in a broad range of applications. In the medical field, fiber-optic illuminators are widely used in endoscopy, and comprise various light sources, fiber-optics, and endoscopes. Bulb-based medical fiber sources are currently widely available.
Light sources and fiber-optics are commonly used for microscopy illumination, with lamp-based products being generally available. Fiber-optic illumination systems are also used in industrial boroscopes and machine vision systems. While the preceding devices primarily provide “white” light for illumination, other fiber-optic light sources providing “blue” light in the wavelength range 420-490 nm are used in photodynamic therapy for pediatric hyperbilirubinemia.
Systems having light sources and fiber-optics for light transmission can also provide one or more defined wavelengths of light for fluorescent excitation in biological and other fields of research.
Many fiber-optic light sources share several common technological limitations. For example, fiber-optics can only accept incoming light rays which lie within an angle determined by the fiber optic materials. For most fiber-optic bundles composed of clad glass fibers, that acceptance half-angle is approximately 33°, corresponding to a numerical aperture (NA) of approximately 0.55. Therefore, for optimal efficiency the fiber-optic light source will usually have some type of focusing optics.
Commonly used fiber-optic bundles composed of clad glass fibers have a transmission factor on the order of 50%-70%. That is, only 50%-70% of the light impinging on the input face of the fiber bundle will exit the fiber optic as useable light. These losses are due to Fresnel losses at the input and output faces, the numerical aperture restriction, the fact that fiber bundles are typically composed of hundreds of small fibers with gaps between them, and attenuation losses along the fiber length. Therefore, fiber optic light sources must provide nearly two times the light that is desired at the fiber output.
For many applications, it is desirable that the light exiting the fiber bundle for illumination be uniform in color and intensity; however, a light source comprised of a bulb or of multiple LEDs may not provide uniform light, particularly in the far field. While fiber bundles can provide some degree of spatial light mixing due to randomization of fibers within the bundle, this is sometimes not sufficient and the fiber light source must use optics so as to produce uniform light from a non-uniform source.
Many currently marketed fiber optic light sources use halogen, metal halide, or xenon bulbs. While these bulbs-based systems can be a cost-effective means to produce white light of sufficient intensity, many have a short (e.g., less than a thousand hour) life; may include toxic materials that require special handling for manufacture and disposal; require high voltages to operate, thereby increasing the cost, size, and safety risk of the power supply; allow the color temperature to be varied only within a narrow range by varying the operating voltage, thereby altering the light intensity; have optional filters to provide different color temperatures, but at the expense of reduced output; generate light over a wide spectrum and thus require optical filters to narrow the wavelengths, which reduces light output and adds cost; and/or emit radiation in the infrared (IR) and ultraviolet (UV) wavelengths, which may have to be blocked with filters or other means, requiring additional optical components.
In response to the aforementioned issues, a number of devices have been manufactured or proposed which use light emitting diodes (LEDs) as light sources for fiber-optic illumination. In general, benefits of LED-based fiber-optic systems include longer (e.g., tens of thousands of hours) life; less and/or no toxic materials; low (e.g., less than 4 volts per LED) voltage; variable color temperature; specific wavelength specification; and very low UV or IR emission. Unfortunately, LED systems have their own unique technical challenges; in particular, limited light output. Most individual LEDs still produce significantly less light than most incandescent lamps. For example, the present maximum light output reasonably achievable from a single 1 mm white LED is approximately 200 lumens, whereas a 300 watt xenon lamp can produce over 2,000 lumens. Therefore, LED fiber sources have to use a plurality of LEDs to produce the desired output light intensity.
Additionally, for an optical system that includes lenses, there is a fundamental law of optics (Etendue) governing the image size and ray angle. For perfect lenses, the product of the image size and the ray angle is a constant (sometimes termed the Lagrangian invariant or optical system invariant). In reality, all lenses have aberrations which increase the value of this constant. In the case of coupling LED light into a fiber, the Etendue principle dictates that the product of the image size and the angle at the fiber face must be greater than the product of the LED source size and the emission angle.
In the context of the Etendue principle, there are additional constraints on the optical design. First, LEDs are fundamentally Lambertian sources which emit light in a hemispherical pattern, wherein the intensity varies as the cosine of the emission angle. Some LED packages include a lens which modifies this pattern. Therefore, the source angle is set by the choice of LED. Second, the size and shape of the LED source is set by the LED manufacturer. In the case of present high-brightness LEDs, the typical size and shape is 1 mm square. Therefore, the source size is set by the choice of LED. Third, the acceptance angle, or numerical aperture (NA), of the fiber optic is a function of fiber core and cladding material. The glass fibers most commonly used have a NA on the order of 0.5, and fiber materials are determined by the end-use application. Therefore, the illuminator design is constrained by the image angle. Fourth, the shape of fiber optics is almost universally circular. (Although other shapes, such as a square, can be achieved by fusing the glass fibers, this is uncommon in practice.) The size, or diameter, of the fiber optic is determined by the end-use application. Therefore, the illuminator design is also constrained by the image shape and size.
Further, a portion of the electrical energy consumed by LEDs generates heat rather than light. Compared to incandescent bulbs, LEDs must operate at much lower temperatures; typically, 120° C.-180° C. While bulbs dissipate heat by infrared emission, heat must be removed from LEDs by conduction from their non-emitting surface. These thermal factors typically impose constraints on the number of LEDs that can be placed closely together in an illumination device.
It is also known that in LED illuminators, light from multiple LEDs is generally needed to achieve the desired total light intensity, as contrasted to a single bulb in conventional illuminators. Individual LEDs, even those from the same manufacturing lot, will not have identical spectral or spatial intensity characteristics, and the LED characteristics are not typically under the optical designer's control. The challenge, then, with LED fiber illuminators is to combine light from multiple non-identical sources and create light which is uniform in color, spatial distribution, and angular distribution.